A petrographic study is presented of three specimens from Malpelo, a volcanic island in the eastern equatorial Pacific Ocean. Two of the specimens show appreciable alteration by phosphatic solutions from guano: an amygdaloid in which the feldspars have been completely destroyed and replaced and a phosphate rock composed essentially of phosphosiderite and strengite, a dimorphous pair with the composition FePO4·2H2O. The third is a comparatively unaltered augite andesite having a rather low silica content and possibly containing anemousite. The petrologic relations among aluminum-iron and calcium phosphates are discussed briefly.

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